1. Field of the Invention
The present invention relates to a vulcanized thread rubber composition and a golf ball using the vulcanized thread rubber composition excellent in rebound characteristics.
2. Description of the Background Art
Golf balls are generally classified into two categories: thread-wound golf balls having a center wound with thread rubber as its core material which is enveloped by a cover; and solid golf balls having a core material of solid rubber which is enveloped by a cover. The thread-wound golf balls are superior in shot feeling and controllability, while the solid golf balls are superior in flight performance and durability.
The thread-wound golf balls exhibiting such favorable controllability have often been employed by advanced amateur golfers and professional golfers. However, ordinary amateur golfers are reluctant to use the thread-wound golf balls because of their inferior flight performance. Thus, improvement of the flight performance of the thread-wound golf balls has been a critical issue.
Conceivable ways to improve the flight performance of a thread-wound golf ball include the one to improve impact resilience of the golf ball and the one to reduce the spin rate thereof. For example, U.S. Pat. No. 5,816,939 discloses a technique to improve the impact resilience, in which a thread rubber for a golf ball having a retention of tensile strength after aging at 70xc2x0 C. for seven days of at least 70%, a hysteresis loss at a 100 kg/cm2 constant stress tensile of at least 50%, and an elongation at the same time of 900% to 1400% is provided.
U.S. Pat. No. 4,353,557 discloses a technique to provide a thread rubber formed of a rubber composition having 2-20 parts by weight of carbon black blended with respect to 100 parts by weight of natural rubber and/or cis 1,4-polyisoprene.
Japanese Patent Publication No. 5-41272 discloses a technique to provide a thread rubber including isoprene-butadiene random copolymer having a cis 1,4 bond content of at least 80 wt % as a rubber component.
A technique to improve the flight performance by reducing the spin rate of the golf ball is disclosed, for example, in U.S. Pat. No. 5,421,580, in which a golf ball having a liquid center whose diameter is 29.5 mm to 32.0 mm and the total capacity of dimples of 280 mm3 to 340 mm3 is provided.
Thread rubber is generally elongated by 800 to 1100% when it is wound around a center during a manufacturing process of a golf ball. When the thread rubber is elongated, rubber molecules tend to be orientated uniformly in the elongated direction, so that crystallization takes place. As a result, rubber elasticity is lost, thereby decreasing the impact resilience of the golf ball. Based on such knowledge, a thread rubber having high impact resilience at the elongated state is highly demanded.
Further, if the thread rubber breaks during the elongated state, productivity is considerably decreased. Thus, prescribed elongation percentage and strength are required for the thread rubber. Still further, prescribed tensile resilience is required for the thread rubber in the elongated state so as to retain sufficient hardness for the golf ball.
The present invention is directed to solve the above-described problems. An object of the present invention is to provide a thread rubber excellent in impact resilience at the elongated state that can suppress breakage during a winding step, and a golf ball employing such thread rubber.
According to an aspect of the present invention, a vulcanized thread rubber composition is obtained by vulcanizing a thread rubber composition including at least 50 wt % of polyisoprene rubber having a cis 1,4 bond content of 80% to 95% and a trans 1,4 bond content of 2% to 4% within a rubber component. In the thread rubber composition, a ratio of a crosslinked density of polysulfide bonds with respect to a total crosslinked density in the vulcanizate is more than 70%.
Preferably, the crosslinked density of the polysulfide bonds is in a range between 0.8xc3x9710xe2x88x928 mol/mm3 and 3xc3x9710xe2x88x928 mol/mm3.
Still preferably, an average cis 1,4 bond content within the rubber component is 90-96% and an average trans 1,4 bond content within the rubber component is 0.5-3.0%.
Preferably, the total crosslinked density of the vulcanized thread rubber composition is between 0.8xc3x9710xe2x88x928 mol/mm3 and 3xc3x9710xe2x88x928 mol/mm3, and the ratio of crosslinked density of the polysulfide bonds is more than 70%.
Still preferably, the vulcanized thread rubber composition is produced by continuous vulcanization, at a pressure of 0.3-1 MPa and a temperature of 140-160xc2x0 C.
Preferably, a ratio of a blended amount of vulcanization accelerator with respect to a blended amount of sulfur (vulcanization accelerator/sulfur) in the rubber composition before vulcanization is less than 2.
According to another aspect of the present invention, a golf ball is obtained by elongating the vulcanized thread rubber composition described above to wind it around a center to form a core material, and enveloping the core material with a cover.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.